US11565347B2 - Laser processing machine - Google Patents

Abstract

A laser processing machine includes a condenser and a water pillar forming unit. The condenser condenses a laser beam emitted from a laser oscillator and irradiates it to a workpiece held on a chuck table. The water pillar forming unit is disposed on a lower end of the condenser and is configured to form a thread-shaped water pillar on a front side of the workpiece. The laser oscillator includes a first laser oscillator, which emits a first laser beam having a short pulse width, and a second laser oscillator, which emits a second laser beam having a long pulse width. After the laser beams emitted from the first and second laser oscillators have transmitted through the thread-shaped water pillar formed by the water pillar forming unit and have been irradiated to the workpiece, a plasma occurred in the water pillar forming unit applies processing to the workpiece.

Description

BACKGROUND OF THE INVENTIONField of the Invention

The present invention relates to a laser processing machine including a chuck table configured to hold a plate-shaped workpiece, a laser beam irradiation unit configured to irradiate a laser beam to the workpiece held on the chuck table so that processing is applied to the workpiece, and a processing feed mechanism configured to perform relative processing feed of the chuck table and the laser beam irradiation unit.

Description of the Related Art

A wafer, on a front side of which a plurality of devices such as integrated circuits (ICs), large scale integrated circuits (LSIs) or the like are formed and defined by a plurality of intersecting streets, is divided into the individual devices by a laser processing machine, and the thus-divided device chips are used in electrical equipment such as mobile phones, personal computers, lighting equipment or the like.

On the other hand, there are a variety of types of laser processing machines, including: a type that irradiates to a workpiece a laser beam of a wavelength having absorption in the workpiece to apply ablation processing to the workpiece so that grooves are formed as starting points for division (see, for example, JP H10-305420 A), another type that irradiates to a workpiece a laser beam of a wavelength having transmissivity through the workpiece with a focal point thereof positioned inside the workpiece, thereby forming a modified layer as a starting point for division inside the workpiece (see, for example, Japanese Patent No. 3408805), and a further type that irradiates to a workpiece a laser beam of a wavelength having transmissivity through the workpiece with a focal point thereof positioned inside the workpiece, thereby forming, as starting points for division, a plurality of shield tunnels each being formed of a fine hole and an amorphous region surrounding the fine hole (see, for example, JP 2014-221483 A). Depending on the kind of a workpiece, a required processing accuracy, and the like, a desired laser processing machine is chosen for use.

In the type that applies ablation processing to a workpiece, debris pieces may scatter around from its sites to which a laser beam is irradiated, and may stick to devices formed on the front side of the workpiece, thereby raising a problem that the devices may be degraded in quality. To prevent such sticking of debris particles, it has hence been also proposed to apply a liquid resin to the front side of a wafer before performing laser processing (see, for example, JP 2004-188475 A).

SUMMARY OF THE INVENTION

If a liquid resin is applied before performing laser processing on a workpiece as described above, the liquid resin is discarded without reuse after the laser processing, leading to problems that such a technique is uneconomical and has a low productivity due to the need for application and removal steps for the liquid resin.

Further, it has also been investigated to prevent debris particles from sticking to the front side of a wafer by irradiating a laser beam to the wafer with the wafer immersed in water and causing the debris particles to float in the water. However, indications have been made about a problem that the laser beam is scattered by bubbles and cavitation occurred in the water, resulting in a failure to perform desired processing. In addition, a further problem has also been found that individually divided chips have a lowered flexural strength due to effects of heat.

The present invention therefore has as an object thereof the provision of a laser processing machine, which can prevent scattering of debris pieces without lowering the productivity and can perform appropriate laser processing without causing scattering of a laser beam.

In accordance with an aspect of the present invention, there is provided a laser processing machine including a chuck table configured to hold a plate-shaped workpiece, a laser beam irradiation unit configured to irradiate a laser beam to the workpiece held on the chuck table so that processing is applied to the workpiece, and a processing feed mechanism configured to perform relative processing feed of the chuck table and the laser beam irradiation unit. The laser beam irradiation unit includes a laser oscillator that emits the laser beam, a condenser that condenses the laser beam emitted from the laser oscillator and irradiates to the workpiece held on the chuck table, and a water pillar forming unit disposed on a lower end of the condenser and configured to form a thread-shaped water pillar on a front side of the workpiece. The laser oscillator includes a first laser oscillator, which emits a first laser beam having a short pulse width, and a second laser oscillator, which emits a second laser beam having a long pulse width, whereby, after the first laser beam emitted from the first laser oscillator and the second laser beam emitted from the second laser oscillator have transmitted in the thread-shaped water pillar and have been irradiated to the workpiece, a plasma produced by the first laser beam emitted from the first laser oscillator grows by absorption of energy from the second laser beam emitted from the second laser oscillator, and applies processing to the workpiece.

Preferably, the water pillar forming unit includes a casing and a pressurized water introducing portion that introduces pressurized water into the casing, the casing having a top wall which faces an object lens configuring the condenser, a bottom wall which opposes the top wall and includes an ejection hole formed therethrough, and a side wall surrounding a space formed by the top wall and the bottom wall, and the first laser beam and the second laser beam are guided by the thread-shaped water pillar ejected from the ejection hole formed through the bottom wall, and are irradiated to the workpiece.

According to the present invention, the sticking of debris particles can be prevented without applying a liquid resin to the front side of a wafer, thereby enabling to cut off the cost of the liquid resin and also to save the labor involved in applying the liquid resin to the front side of the wafer.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.1 is a perspective view of a laser processing machine according to an embodiment of the present invention;

FIG.2 is an exploded perspective view of the laser processing machine depicted inFIG.1, with a portion of the laser processing machine being depicted in an exploded fashion;

FIG.3A is a perspective view of a water pillar forming unit mounted on the laser processing machine depicted inFIG.1;

FIG.3B is an exploded perspective view depicting, in an exploded fashion, the water pillar forming unit depicted inFIG.3A;

FIG.4 is a block diagram for describing an optical system of a laser beam irradiation unit mounted on the laser processing machine depicted inFIG.1;

FIG.5 is a partly enlarged cross-sectional diagram illustrating an operation state of the water pillar forming means, which is mounted on the laser processing machine depicted inFIG.1, at the time of processing;

FIG.6 is a timing chart schematically illustrating the pulse widths of first and second laser beams and also illustrating irradiation timings;

FIG.7A is a partly enlarged cross-sectional view illustrating plasmas occurred when processing is applied to a wafer by the laser beams illustratedFIG.5; and

FIG.7B is a partly enlarged cross-sectional view illustrating a processed groove obtained as a result of the processing inFIG.7A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the attached drawings, a description will hereinafter be made in detail about a laser processing machine according to an embodiment of the present invention.FIG.1 is a perspective view of thelaser processing machine2 of this embodiment. Thelaser processing machine2 includes a water supply system4 disposed on abed21 and configured to supply water onto a plate-shaped workpiece (for example, a silicon-made wafer10), a laserbeam irradiation unit8 configured to irradiate a laser beam to the plate-shaped workpiece, aholding unit22 configured to hold the workpiece, amoving mechanism23 configured to perform a relative movement of the laserbeam irradiation unit8 and theholding unit22, and aframe body26 formed from avertical wall portion261, which is disposed upright in a Z direction indicated by an arrow Z on thebed21 beside themoving mechanism23, and ahorizontal wall portion262, which extends in a horizontal direction from an upper end portion of thevertical wall portion261.

Inside thehorizontal wall portion262 of theframe body26, an optical system which will be described in detail subsequently herein is accommodated. The optical system configures the laserbeam irradiation unit8 that irradiates a laser beam to thewafer10 held on theholding unit22. On the side of a lower surface of a distal end portion of thehorizontal wall portion262, acondenser86 that configures a portion of the laserbeam irradiation unit8 is disposed, and analignment unit90 is also disposed at a location adjacent thecondenser86 in an X direction indicated by an arrow X in the figure.

Thealignment unit90 is used to image thewafer10 held on a chuck table34, which makes up theholding unit22, and to detect a position where laser processing is to be applied. Thealignment unit90 includes an imaging element (charge-coupled device (CCD)) that uses a visible beam to image the front side of thewafer10. Depending on the material that forms thewafer10, thealignment unit90 may preferably include infrared ray irradiation means for irradiating an infrared ray, an optical system that captures the infrared ray irradiated by the infrared ray irradiation means and reflected on the front side of thewafer10, and an imaging element (infrared CCD) that outputs an electrical signal corresponding to the infrared ray captured by the optical system.

As depicted in the figure, thewafer10 is supported, for example, on an annular frame F via an adhesive table T, and is held under suction on asuction chuck35 that makes up an upper surface of the chuck table34. The above-describedlaser processing machine2 is covered in its entirety by a housing or the like a depiction of which is omitted in the figure for the convenience of description, and is configured to prevent penetration of fine powder, dust, and the like thereinto.

Referring toFIG.2 in addition toFIG.1, a description will be made in detail about thelaser processing machine2 according to this embodiment.FIG.2 is a perspective view depicting thelaser processing machine2 ofFIG.1, in which awater recovery pool60 that configures a section of the water supply system4 has been detached from thelaser processing machine2 and a part of the detached section is depicted in an exploded fashion.

Theholding unit22 includes, as depicted inFIG.2, a rectangularX-direction moving plate30 mounted on thebed21 movably in the X direction indicated by the arrow X, a rectangular Y-direction moving plate31 mounted on theX-direction moving plate30 movably in a Y direction indicated by an arrow Y and intersecting the X direction at right angles, acylindrical post32 fixed on an upper surface of the Y-direction moving plate31, and arectangular cover plate33 fixed on an upper end of thepost32. On thecover plate33, the chuck table34 is disposed extending upward through an elongated hole formed in thecover plate33. The chuck table34 is configured to hold thewafer10 and to be rotatable by undepicted rotary drive means. Configuring the upper surface of the chuck table34, thesuction chuck35 is disposed. Thesuction chuck35 has a circular shape, is formed from a porous material having air permeability, and extends substantially horizontally. Thesuction chuck35 is connected to undepicted suction means via a flow passage that extends through thepost32, and fourclamps36 are arranged at intervals around thesuction chuck35. Theclamps36 grip the frame F with thewafer10 held thereon when fixing thewafer10 on the chuck table34. A plane defined by the X direction and the Y direction forms a substantially horizontal plane.

Themoving mechanism23 includes anX-direction moving mechanism50 and a Y-direction moving mechanism52. TheX-direction moving mechanism50 converts a rotary motion of amotor50ato a linear motion via aball screw50b, and transmits the linear motion to theX-direction moving plate30, whereby theX-direction moving plate30 is advanced or retracted in the X direction alongguide rails27 on thebed21. The Y-direction moving mechanism52 converts a rotary motion of amotor52ato a linear motion via aball screw52b, and transmits the linear motion to the Y-direction moving plate31, whereby the Y-direction moving plate31 is advanced or retracted in the Y direction alongguide rails37 on theX-direction moving plate30. Although not depicted in the figures, the chuck table34, theX-direction moving mechanism50 and the Y-direction moving mechanism52 each include position detecting means, and therefore positions of the chuck table34 in the X and Y directions and an angular position of the chuck table34 in a peripheral direction are detected accurately. TheX-direction moving mechanism50, the Y-direction moving mechanism52 and the undepicted rotary drive means for the chuck table34 are then driven, thereby enabling to accurately position the chuck table34 at desired positions and angle. The above-describedX-direction moving mechanism50 acts as processing feed means for moving theholding unit22 in a processing feed direction, and the above-described Y-direction moving mechanism52 acts as indexing feed means for moving theholding unit22 in an indexing feed direction.

As depicted inFIG.1, the water supply system4 includes a waterpillar forming unit40 that configures water pillar forming means in this embodiment, awater supply pump44, afilter45, awater recovery pool60, apipe46aconnecting the waterpillar forming unit40 and thewater supply pump44 together, and apipe46bconnecting thewater recovery pool60 and thefilter45 together. Preferably, thepipe46aand thepipe46bmay each be formed by a flexible hose in its part or entirety.

As depicted inFIG.3A, the waterpillar forming unit40 is disposed on a lower end portion of thecondenser86. An exploded view of the waterpillar forming unit40 is presented inFIG.3B. As appreciated fromFIG.3B, the waterpillar forming unit40 includes acasing42 and a pressurizedwater introducing portion43 that introduces pressurized water into thecasing42. Thecasing42 has a substantially rectangular shape as seen in plan, and is configured of anupper member421, which makes up a top wall in this embodiment, and alower member422 opposing theupper member421.

As depicted inFIG.3B, acircular opening421cis formed through theupper member421, extending from the side of anupper surface421ato the side of aback surface421b. Thecondenser86 is inserted in theopening421c. Thelower member422 includes abottom wall422dand aside wall422c. Thebottom wall422dis located in a region that opposes theopening421cof theupper member421, and has anejection hole423 formed therethrough extending to the side of alower surface422bof thelower member422. Theside wall422csurrounds aspace424 formed by thelower surface421bof theupper member421 and thebottom wall422d. In aside wall422fon a side where the pressurizedwater introducing portion43 is connected, awater supply port422eis formed to supply water to thespace424.

The pressurizedwater introducing portion43 includes asupply port43athrough which water W is supplied, and the waterpillar forming unit40 is formed by assembling the pressurizedwater introducing portion43 on theside wall422f, in which thewater supply port422eopens, of thecasing42 from the Y direction.

The waterpillar forming unit40 has such a configuration as described above, and the water W delivered from thewater supply pump44, which has been described based onFIG.1, is supplied to thewater supply port422eof thecasing42 via the pressurizedwater introducing portion43, is allowed to flow through thespace424 of thecasing42, and is ejected from theejection hole423 formed in thebottom wall422d.

Referring back toFIGS.1 and2, a description will be made about thewater recovery pool60. As depicted inFIG.2, thewater recovery pool60 includes an outer frame member61 and two water covers66.

The outer frame member61 includes a pair ofouter side walls62aextending in the X direction indicated by the arrow X in the figures, a pair ofouter side walls62bextending in the Y direction indicated by the arrow Y in the figures, pairs ofinner side walls63aand63bdisposed inside, with a predetermined interval from and in parallel to theouter side walls62aand62b, and abottom wall64 connecting theouter side walls62aand62bandinner side walls63aand63btogether at lower ends thereof. Theouter side walls62aand62b, theinner side walls63aand63b, and thebottom wall64 form a rectangularwater recovery channel70, which has long sides extending along the X direction and short sides extending along the Y direction. Inside theinner side walls63aand63bthat form thewater recovery channel70, an opening is formed extending upward and downward. Thebottom wall64, which forms thewater recovery channel70, has a slight inclination in the X direction and the Y direction. At a corner portion located at the lowest position of the water recovery channel70 (the left corner portion in the figure), awater drain hole65 is disposed. Thepipe46bis connected to thewater drain hole65, so that thewater drain hole65 is connected to thefilter45 via thepipe46b. The outer frame member61 may preferably be formed in its entirety from a stainless steel plate resistant to corrosion and rusting.

The two water covers66 each include a resin-madecorrugated cover member66b, and fixingfittings66ahaving a flattened square U-shape and fixedly secured on opposite ends of thecover member66b, respectively. The fixingfittings66aare formed with dimensions sufficient to straddle the twoinner side walls63aof the outer frame member61, the twoinner side walls63abeing disposed opposing each other in the Y direction. One of the fixingfittings66aof eachwater cover66, specifically the outer fixing fitting66aas viewed in the X direction is fixed on theinner side wall63bof the outer frame member61, theinner side wall63bopposing the outer fixing fitting66ain the X direction. Thewater recovery pool60 configured as described above is fixed on thebed21 of thelaser processing machine2 by undepicted fixing means. Thecover plate33 of the holdingunit22 is attached such that thecover plate33 is held between theinner fixing fittings66aof the two water covers66. X-direction end faces of thecover member33 have the same fattened square U-shape as theinner fixing fittings66a, and similar to theinner fixing fittings66a, are dimensioned to straddle theinner side walls63aof the outer frame member61 in the Y direction. Therefore, thecover member33 is attached to the water covers66 after the outer frame member61 of thewater recovery pool60 has been disposed on thebed21. According to the above-described configuration, thecover plate33 moves along theinner side walls63aof thewater recovery pool60 when thecover plate33 is moved in the X direction by theX-direction moving mechanism50. However, the manner of attachment of the water covers66 and thecover member33 is not limited to the above-described procedures. For example, thecover member33 may be attached beforehand to the water covers66 prior to the attachment of the two water covers66 to theinner side walls63bof the outer frame member61, and the water covers66 may then be attached together with thecover member33 to the outer frame member61 attached beforehand to thebed21.

Continuing the description with reference toFIG.1 again, the water supply system4 has the above-described configuration so that the water W delivered from adelivery port44aof thewater supply pump44 is supplied to the waterpillar forming unit40 by way of thepipe46a. The water W supplied to the waterpillar forming unit40 is ejected downward from theejection hole423 formed through thebottom wall422dof thecasing42 of the waterpillar forming unit40 described based on FIG.3B. The water W ejected from the waterpillar forming unit40 flows on thecover plate33 and the water covers66, and flows down into thewater recovery pool60. The water W, which has flowed down into thewater recovery pool60, flows through thewater recovery channel70, and is collected in thewater drain hole65 disposed at the lowest position of thewater recovery channel70. The water W collected in thewater drain hole65 is guided to thefilter45 by way of thepipe46b, and subsequent to removal of laser swarf (debris particles), fine powder, dust, and the like at thefilter45, is returned to thewater supply pump44. Therefore, the water W delivered from thewater supply pump44 circulates in the water supply system4.

FIG.4 is a block diagram illustrating the outline of the optical system of the laserbeam irradiation unit8. As illustrated inFIG.4, the laserbeam irradiation unit8 includes alaser oscillator81, a half-wave plate82, a half-wave plate84, apolarizing beam splitter85, areflection mirror87, and thecondenser86. Thelaser oscillator81 includes afirst laser oscillator812 that emits a first laser beam LB1 as a pulsed laser beam having a short pulse width, and asecond laser oscillator814 that emits a second laser beam LB2 as a pulsed laser beam having a long pulse width. The half-wave plate82 imparts a phase difference of ½ wavelength to the first laser beam LB1 entered thereinto, and rotates the polarization direction of a linearly polarized laser beam. The half-wave plate84 imparts a phase difference of ½ wavelength to the second laser beam LB2 entered thereinto, and rotates the polarization direction of a linearly polarized laser beam. Thepolarizing beam splitter85 reflects an S-polarized component of the first laser beam LB1 passed through the half-wave plate82, allows a P-polarized component of the second laser beam LB2 passed through the half-wave plate84, combines the reflected S-polarized component of the first laser beam LB1 and the passed P-polarized component of the second laser beam LB2 so as to irradiate them to the same point on thewafer10, and outputs them as a laser beam LB1+LB2. Thereflection mirror87 changes by 90° the irradiation direction of the laser beam LB1+LB2 outputted from thepolarizing beam splitter85. Thecondenser86 condenses the laser beam LB1+LB2, and irradiates it to thewafer10 held on the holdingunit22. Thefirst laser oscillator812 and thesecond laser oscillator814 oscillate, for example, lasers of wavelengths having absorption in thewafer10. Although not illustrated in the figure, the optical system of the laserbeam irradiation unit8 may also include attenuators to change the outputs of the individual laser beams, reflection mirrors to change the optical paths of the individual laser beams, and the like, as needed.

Inside thecondenser86, anobject lens86ais disposed to focus and irradiate the laser beam LB1+LB2 to thewafer10. Theobject lens86ais located below the above-describedreflection mirror87, and condenses and irradiates the laser beam LB1+LB2 reflected by thereflection mirror87. On the lower end portion of thecondenser86, aglass member86bis disposed so that the lower end portion of thecondenser86 is closed by theglass member86b. Theglass member86btransmits the above-described laser beam LB1+LB2, and prevents the pressurized water W, which is introduced into thespace424 of thecasing42, from entering thecondenser86. Instead of closing the lower end portion of thecondenser86 by theglass member86b, theopening421cformed in theupper member421 of thecasing42 may be closed by a similar glass member on the side of a lower surface of theupper member421. In addition, the laserbeam irradiation unit8 includes unillustrated focal point position adjusting means, and adjusts in the Z direction the position of the focal point of the laser beam LB1+LB2 focused by thecondenser86.

Thelaser processing machine2 according to the present invention has the configuration as generally described above, and its operations will be hereinafter described. When performing laser processing by thelaser processing machine2 of this embodiment, a plate-shaped workpiece, for example, thewafer10, which is made from silicon (Si) and carries devices formed on the front side thereof, supported on the annular frame F via the adhesive tape T as depicted inFIG.1 is provided. After thewafer10 has been provided, thewafer10 is placed, with the front side with the devices formed thereon facing upward, on thesuction chuck35 of the chuck table34, the undepicted suction means is operated, and the annular frame F is fixed by theclamps36 or the like.

After thewafer10 has been held on thesuction chuck35, the chuck table34 is moved in the X and Y directions by the movingmechanism23 as needed to position thewafer10 on the chuck table34 right below thealignment unit90. After thewafer10 has been positioned right below thealignment unit90, thewafer10 is imaged from above by thealignment unit90. Based on an image of thewafer10 as captured by thealignment unit90, a position on thewafer10, where processing is to be performed, is next detected by a method such as pattern matching (alignment step). By moving the chuck table34 based on position information acquired by this alignment step, thecondenser86 is positioned above a processing start position on thewafer10.

After the alignment between thecondenser86 and thewafer10 has been performed, the water W is replenished to the water supply system4 as needed and sufficiently, and thewater supply pump44 is operated. As the water W that circulates in the water supply system4, pure water may be used, for example.

FIG.5 presents a partly enlarged cross-sectional view of the waterpillar forming unit40 sectioned along A-A indicated inFIG.3, and also illustrates a mode that performs laser processing by irradiating the laser beam LB1+LB2 while introducing the pressurized water W and forming a water pillar Wp. As appreciated fromFIG.5, the waterpillar forming unit40 of the water supply system4 is disposed on the lower end portion of thecondenser86, and is set so that, when the focal point of the laser beam LB1+LB2 is positioned at the height of the front side of thewafer10, a water pillar forming region S of, for example, approximately 10 to 20 mm high is formed between thelower surface422bof thecasing42 of the waterpillar forming unit40 and the front side of thewafer10.

As the water supply system4 has the above-described configuration, the water W delivered from thedelivery port44aof thewater supply pump44 is supplied to the waterpillar forming unit40. The water W supplied to the waterpillar forming unit40 is introduced into thespace424 of thecasing42 via the pressurizedwater introducing portion43, and is ejected downward from theejection hole423 formed through thebottom wall422d. The water W ejected from theejection hole423 forms the thread-shaped water pillar Wp in the water pillar forming region S between thelower surface422bof thecasing42 and thewafer10 as illustrated inFIG.5. Thereafter, the water W flows on thewafer10 and flows out of the chuck table34, flows through the above-describedwater recovery channel70 of thewater recovery pool60, and is collected in thewater drain hole65 formed in thewater recovery channel70. The water W collected in thewater drain hole65 is guided to thefilter45 by way of thepipe46b, is cleaned at thefilter45, is returned to thewater supply pump44, and is allowed to circulate in the water supply system4. Thespace424 filled with the water W delivered from thewater supply pump44 has a pressure of 2 to 50 MPa, for example, and the thread-shaped water pillar Wp formed with the water W ejected from theejection hole423 has a diameter of 20 to 150 μm, for example.

With the water W stably circulating in the water supply system4 and the water pillar Wp being formed, the laserbeam irradiation unit8 is operated and theX-direction moving mechanism50, which configures the movingmechanism23, is operated, whereby the holdingunit22 and the laserbeam irradiation unit8 are relatively moved at a predetermined moving speed in the processing feed direction (the X direction) from the above-described processing start position.

Now, a description will be made in further detail, with reference toFIGS.6,7A and7B in addition toFIG.5, about laser processing to be realized by the laserbeam irradiation unit8 of this embodiment. As illustrated inFIG.5, the laser beam LB1+LB2 irradiated from thecondenser86 passes through thespace424, which is filled with the water W, and theejection hole423 of the waterpillar forming unit40, is allowed to transmit in the water pillar Wp, and is irradiated to a position (a desired street) on thewafer10, where the processing is to be performed. The laser beam LB1+LB2 is the combination of the first laser beam LB1 and the second laser beam LB2 as described above. As illustrated inFIG.6, the first laser beam LB1 is set to have an extremely short pulse width A, the second laser beam LB2 is set to have a pulse width B longer than the first laser beam LB1, and the second laser beam LB2 is irradiated so as to synchronize with the first laser beam LB1.

The above-described laser processing conditions for thelaser processing machine2 can be realized, for example, under the following specific processing conditions.

<First Laser Oscillator>

Wavelength of the first laser beam: 355 nm, 532 nm, 1064 nm

Average output: 10 to 30 W

Repetition frequency: 1 to 10 MHz

Pulse width: 50 fs to 50 ps

<Second Laser Oscillator>

Wavelength of the second laser beam: 355 nm, 532 nm, 1064 nm

Average output: 30 W

Repetition frequency: 1 to 10 MHz

Pulse width: 50 ns

As appreciated fromFIGS.6 and7A, the second laser beam LB2 is irradiated so as to be introduced into a first plasma P1 occurred in a vicinity of the front side of thewafer10 as a result of irradiation of the first laser beam LB1 to the position of thewafer10 where processing is to be performed. In this embodiment, as described based onFIG.6, the first laser beam LB1 is set have the extremely short pulse width, and the second laser beam LB2 is set to have the pulse width longer than the first laser beam LB1. In other words, the first laser beam LB1 is set have a high peak power density, and the second laser beam LB2 is set to have a peak power density significantly lower than the first laser beam LB1.

If the laser beam LB1+LB2 is irradiated to thewafer10 as described above, the first plasma P1 occurs in the vicinity of the front side of thewafer10 as the result of the irradiation of the first laser beam LB1 of the high peak power density and short pulse width as illustrated inFIG.7A. Further, the second laser beam LB2 is irradiated toward the first plasma P1 because the second laser beam LB2 is irradiated so as to synchronize with the first laser beam LB1. As a consequence, the energy of the second laser beam LB2 is guided to the first plasma P1, and therefore the first plasma P1 is allowed to grow into a second plasma P2. The laser beam LB1+LB2 is then irradiated along the desired street, and as illustrated inFIG.7B, laser processing that is excellent in isotropy is applied toward a location underneath the irradiation position, whereby thewafer10 is processed deeper to form a fine cylindrical hole. Then, by moving the holdingunit22 in the processing feed direction (the X direction), the processedgroove100 is formed to a desired depth along the desired street. During this processing, bubbles occur by the irradiation of the laser beam LB1+LB2. However, these bubbles are caused to collapse by the pressure of the water pillar Wp, are promptly expelled together with the water W of the water pillar Wp from the processing region for thewafer10, and therefore do not interfere with the intermittently irradiated laser beam LB1+LB2.

Even if debris particles are released from the front side of thewafer10 into the water W, such debris particles are promptly expelled together with the above-described bubbles. The water W with the above-described bubbles and debris particles contained therein flows on thecover plate33 and the water covers66, and is guided into thewater recovery channel70 of thewater recovery pool60, as mentioned above. The water W guided into thewater recovery channel70 flows through thewater recovery channel70 while externally releasing the bubbles occurred by the laser processing, and is drained from thewater drain hole65 formed at the deepest portion of thewater recovery channel70. The water W drained from thewater drain hole65 is guided to thefilter45 by way of thepipe46b, and is again supplied to thewater supply pump44. Since the water W circulates in the water supply system4 as described above, debris particles, fine powder, dust, and the like are appropriately captured by thefilter45, and hence the water W is maintained in a clean state.

After the above-described laser processing has been performed along the desired street in the first direction, the movingmechanism23 is operated to position thecondenser86 above one end portion of another street which is adjacent in the Y direction the desired street already subjected to the laser processing and has not been processed, and laser processing similar to the above-described laser processing is performed. After such laser processing has been performed on all the streets formed in the first direction, the chuck table34 is rotated by 90°, and similar laser processing is also performed on streets which extend in the second direction, intersect at right angles with the already-processed streets in the first direction, and have not been processed yet. In the manner as described above, laser processing can be performed along all the streets on thewafer10 to form the processedgrooves100 as starting points for division.

In this embodiment, as described above, the laser beam LB1+LB2 is allowed to transmit in the water pillar Wp and is irradiated to the desired irradiation position, whereby processing is applied by the second plasma P2 grown from the first plasma P1. If processing is applied only by a laser beam having a short pulse width like the first laser beam LB1, on the other hand, there is anisotropy in the processing direction, so that the processed portion has a V-shaped cross-sectional shape and the processing speed abruptly decreases as the processing proceeds in a depth direction from the front side. If the laser beam LB1+LB2 as the combination of the first laser beam LB1 having the short pulse width and the second laser beam LB2 having the long pulse width is irradiated as in this embodiment, however, the processing is excellent in isotropy. As described based onFIG.7, thewafer10 is hence processed deeper downwardly of the irradiation position to form a fine cylindrical hole without a reduction in processing speed. In this manner, the processedgroove100 can be formed at a preferred processing speed to a desired depth along each street.

According to this embodiment, the sticking of debris particles to the front side of thewafer10 can be prevented without applying a liquid resin to the front side of the wafer, thereby enabling to cut off the cost of the liquid resin and also to save the labor involved in applying and removing the liquid resin and hence to make improvements in productivity.

Further, the first laser beam LB1 is allowed to transmit in the water pillar Wp formed by the waterpillar forming unit40, and is irradiated to thewafer10 so that the first plasma P1 occurs. At this time, the first plasma P1 occurs confined in a layer of the water W that is flowing down as the water pillar Wp, so that the first plasma P1 is suppressed from excessively spreading and effects of heat are alleviated. On the other hand, the second laser beam LB2 is absorbed in the first plasma P1 produced by the first laser beam LB1 having the short pulse width, so that the second plasma P2 occurs in the layer of the water W, which is flowing down as the water pillar Wp, and applies processing. Compared with the case in which laser processing is performed only by the second laser beam LB2, the effects of heat as applied to surroundings of each street on thewafer10 are limited, leading to improvements in flexural strength when thewafer10 is divided into individual device chips. By combining the first laser beam LB1 and the second laser beam LB2, allowing the laser beam LB1+LB2 to transmit in the water pillar Wp and irradiating the laser beam LB1+LB2 to thewafer10 as in this embodiment, excellent laser processing is feasible compared with a case in which one of the first laser beam LB1 and the second laser beam LB2 is irradiated alone to perform laser processing.

According to the present invention, a variety of modifications can be provided without being limited to the above-described embodiment. In this embodiment, the description is made with a premise that the second laser beam LB2 is, for example, a pulsed laser beam, but the present invention is not limited to such a premise. The second laser beam LB2 may be a laser beam of continuous wave (CW), because it is needed for the second laser beam LB2 to be a laser beam irradiated with a pulse width longer than the pulse width of the first laser beam LB1. Therefore, laser beams of continuous wave (CW) are encompassed by the term “second laser beam having a long pulse width” as used in this invention.

In the embodiment described above, the second laser beam LB2 is outputted so as to synchronize with the first laser beam LB1. Further, as illustrated inFIG.6, the first laser beam LB1 and the second laser beam LB2 are described to be emitted from thefirst laser oscillator812 and thesecond laser oscillator814, respectively, so that the second laser beam LB2 is outputted concurrently with the first laser beam LB1. However, the present invention is not limited to such a mode. For example, the present invention embraces a mode that irradiates the second laser beam LB2 after irradiation of the first laser beam LB1 and before extinction of the first plasma P1 produced by the first laser beam LB1. As described above, even after the first laser beam LB1 has been irradiated, the emission of the second laser beam LB2 from thesecond laser oscillator814 before the extinction of the first plasma P1 can bring about similar advantageous effects as those described above.

The present invention is not limited to the details of the above-described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.

Claims (6)

What is claimed is:

1. A laser processing machine comprising:

a chuck table configured to hold a plate-shaped workpiece,

a laser beam irradiation unit configured to irradiate a laser beam to the workpiece held on the chuck table so that processing is applied to the workpiece, and

a processing feed mechanism configured to perform relative processing feed of the chuck table and the laser beam irradiation unit,

wherein the laser beam irradiation unit includes:

a laser oscillator that emits the laser beam,

a condenser that condenses the laser beam emitted from the laser oscillator and irradiates to the workpiece held on the chuck table,

a water pillar forming unit disposed on a lower end of the condenser and configured to form a thread-shaped water pillar on a front side of the workpiece,

a water recovery pool configured to collect water, and

a water supply pump configured to supply water from the water recovery pool to the water pillar forming unit,

wherein the laser oscillator includes a first laser oscillator, which emits a first laser beam having a short pulse width, and a second laser oscillator, which emits a second laser beam having a long pulse width,

wherein, after the first laser beam emitted from the first laser oscillator and the second laser beam emitted from the second laser oscillator have transmitted through the thread-shaped water pillar and have been irradiated to the workpiece, a plasma produced by

the first laser beam emitted from the first laser oscillator grows by absorption of energy from the second laser beam emitted from the second laser oscillator, and applies processing to the workpiece, and

wherein the water recovery pool collects excess water from the water pillar forming unit.

2. The laser processing machine according toclaim 1, wherein:

the water pillar forming unit includes a casing and a pressurized water introducing portion that introduces pressurized water into the casing, the casing having a top wall which faces an object lens configuring the condenser, a bottom wall which opposes the top wall and includes an ejection hole formed therethrough, and a side wall surrounding a space formed by the top wall and the bottom wall, and

the first laser beam and the second laser beam are guided by the thread-shaped water pillar ejected from the ejection hole formed through the bottom wall, and are irradiated to the workpiece.

3. The laser processing machine according toclaim 1, wherein said water recovery pool includes an outer frame and a drain hole, said outer frame defining a channel that directs the water to the drain hole.

4. The laser processing machine according toclaim 3, wherein said drain hole is connected to a filter configured to clean the water.

5. The laser processing machine according toclaim 3, wherein said water recovery pool includes two covers, said chuck table being positioned between said two covers.

6. A laser processing machine comprising:

a chuck table configured to hold a plate-shaped workpiece,

a laser beam irradiation unit configured to irradiate a laser beam to the workpiece held on the chuck table so that processing is applied to the workpiece, and

a processing feed mechanism configured to perform relative processing feed of the chuck table and the laser beam irradiation unit,

wherein the laser beam irradiation unit includes:

a laser oscillator that includes a first laser oscillator, which emits a first laser beam having a short pulse width, and a second laser oscillator, which emits a second laser beam having a long pulse width,

a beam splitter that combines the first laser beam and the second laser beam and outputs the first laser beam and the second laser beam as the laser beam,

a condenser that condenses the laser beam emitted from the laser oscillator and combined by the beam splitter and irradiates to the workpiece held on the chuck table,

a water pillar forming unit disposed on a lower end of the condenser and configured to form a thread-shaped water pillar on a front side of the workpiece,

a water recovery pool configured to collect water, and

a water supply pump configured to supply water from the water recovery pool to the water pillar forming unit,

wherein, after the first laser beam emitted from the first laser oscillator and the second laser beam emitted from the second laser oscillator have transmitted through the thread-shaped water pillar and have been irradiated to the workpiece, a plasma produced by the first laser beam emitted from the first laser oscillator grows by absorption of energy from the second laser beam emitted from the second laser oscillator, and applies processing to the workpiece.

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